The present invention relates to vehicle safety and, in particular, it concerns a system for helping to prevent skid and roll-over of road vehicles.
Speed of vehicles is increasing along with development of automobile technology and speeding is one of the most prevalent factors contributing to traffic crashes. According to National Highway Traffic Safety Administration (USA) the economic cost to society of speeding related crashes is estimated to be $40.2 billion per year in USA alone. In the year 2004, speeding was identified as a contributory factor in 30% of all fatal crashes and 13,192 lives were lost. Speeding crashes occur when an automobile is driven too fast for the prevalent conditions, more so during winters and on wet/snowy/icy road conditions.
Rollovers are another area of concern, more so due to the rapid growth of sport-utility vehicles (SUVs) in the passenger vehicle segment. According to National Highway Traffic Safety Administration (USA) research note issued in November 2005, 10,553 fatalities occurred due to passenger vehicle rollovers during the year 2004 in USA alone. SUVs, a rapidly growing segment in passenger vehicles, have the highest rate of rollovers. In some of the vehicles, even when they are equipped with ABS and stability control systems, the percentage chance of rollover is as high as 17%.
Two technologies which have been widely adopted in an attempt to improve vehicle controllability are the “anti-locking braking system” (“ABS”) and the “electronic stability control” (“ESC”) system.
ABS (anti-locking braking system) is a computerized system that keeps wheels from locking up during hard brake applications. ABS does not decrease or increase normal braking capability. It also does not necessarily shorten stopping distance, but it helps keeping vehicle under control during hard braking.
ESC (Electronic stability control system) compares the driver's intended direction in steering and braking inputs, to the vehicle's response, via lateral acceleration, rotation (yaw) and individual wheel speeds. ESC then brakes individual front or rear wheels and/or reduces excess engine power as needed to help correct understeer (plowing) or oversteer (fishtailing). ESC also integrates all-speed traction control, which senses drive-wheel slip under acceleration and individually brakes the slipping wheel or wheels, and/or reduces excess engine power, until control is regained. ESC cannot override a car's physical limits. If a driver pushes the possibilities of the car's chassis and ESC too far, ESC cannot prevent a crash. It is a tool to help the driver maintain control.
ESC combines ABS, traction control and yaw control (yaw is spin around a vertical axis). To grasp how it works, think of steering a canoe. If you want the canoe to turn or rotate to the right, you plant the paddle in the water on the right to provide a braking moment on the right side. The canoe pivots or rotates to the right. ESC fundamentally does the same to assist the driver.
Numerous international studies have confirmed the effectiveness of ESC in helping the driver maintain control of the car, help save lives and reduce the severity of crashes. In the fall of 2004 in the U.S., the NHTSA confirmed the international studies, releasing results of a field study in the U.S. of ESC effectiveness. NHTSA concluded that ESC reduces crashes by 35%.
Despite ABS, ESC and traction control systems, an unacceptable number of speeding accidents occur as the drivers push the car too far beyond a threshold point. This threshold point is dynamic as it is defined by the automobile, weather and road conditions and very few experienced drivers can judge this accurately.
It has been proposed to measure vibrations of wheels during operation of a vehicle in order to provide diagnostic information. For example, vibrations at the rate of rotation of the wheel may indicate an imbalance of the wheel. Vibrations at twice that frequency may indicate wear in the CV joint. Vibrations at other frequencies may indicate misfiring of an engine cylinder or the like. A sudden reduction in vibrations may indicate aquaplaning.
Although in principle, the idea of measuring vibrations is promising, the simplistic approach mentioned above is ineffective for deriving reliable information about the interaction of a vehicle with the road. Many different factors may cause variations in the frequency and amplitude of wheel vibrations, including variations in road surface, loading of the vehicle, shift of center of gravity during acceleration or braking. Simple one-dimensional vibration measurements are unable to distinguish between these different causes.
In the field of vector calculus, various parameters may be used to quantify properties of a motion. Of particular relevance to the present invention are vector curvature and vector torsion. Expressed in terms of a point moving with velocity v and acceleration a in three dimensions, the vector curvature is defined by:
      κ    ′    =                                    v          ⇀                ×                  a          ⇀                                                            v          ⇀                            3      and gives a measure of how sharply the path of the point turns in three dimensional space. The vector curvature becomes zero if the acceleration is along the line of the velocity. The vector torsion is defined by:
  τ  =                    (                              v            ⇀                    ×                      a            ⇀                          )            ·                        a          ⇀                ′                                                          a            ⇀                    ×                                    a              ⇀                        ′                                      2      where a′ is the time derivative of the acceleration. The vector torsion is a measure of the helical progression of the motion in three dimensions, and is zero for any path contained in a plane. Parenthetically, it will be noted that both the curvature and the torsion are scalar quantities. They are referred to herein as “vector curvature” and “vector torsion” to identify them as the quantities of curvature and torsion defined in vector calculus, as distinct from other common uses of the terms “curvature” and “torsion”.
There is therefore a need for a system and method based on wheel vibration measurements which would reliably detect impending likelihood of skid or roll-over of a vehicle and would advise or automatically implement preventative correction.